Xylene isomers are important intermediates, which find wide and varied applications in chemical syntheses. By way of example, para-xylene (PX) is a feedstock for terephthalic acid which finds use in the manufacture of synthetic fibers; meta-xylene (MX) is used in the manufacture of dyes; and ortho-xylene (OX) is used as a feedstock for phthalic anhydride which finds use in the manufacture of plasticizers.
C8 aromatics, such as xylenes are found in various fractions from the petrochemical industry, such as coal tar distillate, petroleum reformates, and pyrolysis liquids in admixture with other compounds of like boiling points. The aromatic components from these materials, such C8 aromatics or even xylenes, are readily separated from non-aromatics by methods such as solvent extraction or distillation.
While difficult to separate due to their similar chemical structures and physical properties and identical molecular weights, there are various methods used for separating C8 isomers, for instance OX is separable from other C8 aromatics by fractional distillation, and PX is separable by fractional crystallization or selective adsorption.
The production of PX in a conventional aromatics complex is energy intensive. This is due in a significant part to the equilibrium limitation on PX concentration imposed by the thermodynamics. Under the typical conditions of 200° C. to 500° C. at which xylenes are processed in a typical petrochemical plant, the thermodynamic equilibrium content calculated based on free energy of formation is often approximately 24 mol % PX, 56 mol % MX, and 20 mol % OX, based on the total amount of xylenes in the feed. Such a relatively low PX equilibrium concentration leads to large amounts of MX and OX recycles which are reprocessed through several energy intensive operations, making PX production a costly practice in terms of energy consumption and capital investments. Present demand for PX is fairly large and is expected to grow in the future. Consequently, a system maximizing PX production in an energy-efficient manner is highly sought after.
A typical process is illustrated in FIG. 1. The feed streams to the system comprise C8+ aromatics and may come from one or more sources, including C8+ reformate 1 (see, for instance, U.S. Pat. No. 7,179,367), C8+ selective toluene disproportionation product 17 (see, for instance, U.S. Pat. No. 7,989,672), C8+ transalkylation product 2 (see, for instance, U.S. Pat. No. 7,663,010), C8+ toluene disproportionation product 15 (see, for instance, U.S. Pat. No. 6,198,013), and C8 aromatics, produced from toluene and/or benzene methylation with methanol (see, for instance, U.S. Application 2011/0092755). These streams typically comprise C8 and heavier aromatics which are processed along with a recycle stream 10 in one or more fractionators 16 for the removal C9+-aromatics (aromatic compounds having nine or more carbon atoms) and, optionally, OX in stream 3, which optionally can be subsequently separated in fractionator 14 into OX overhead 4 and C9+ bottoms 5. The C9+-aromatics could have adverse effects on downstream PX Recovery 12 and vapor-phase xylenes isomerization unit 13 if not removed from the feed stream(s) as bottoms by the aforementioned fractionation unit 16 and, optionally, 14.
The removal of C9+-aromatics and, optionally, OX in fractionator(s) 16 thus yields an overhead of C8-aromatics-rich stream 6 which typically contains PX at a concentration of below or near the thermodynamic equilibrium concentration. The C8-aromatics-rich stream 6 is processed to selectively recover PX by one or both of selective adsorption or crystallization which is shown as PX recovery 12. The PX product stream 7 typically having more than 99.7 wt % PX is recovered, and the balance of C8 aromatics stream 8 passes to vapor-phase xylenes isomerization 13. Usually, in the presence of hydrogen in stream 9, vapor-phase xylenes isomerization 13 generates an isomerate (i.e., isomerization product) stream 19 having near-equilibrium concentration of xylene isomers using one or more of a variety of catalysts which may also convert EB to benzene and ethane or may convert EB to near-equilibrium xylene isomers. The isomerate, or isomerization product stream 19, passes to detoluenization fractionation 18 which removes C7-hydrocarbons (hydrocarbon compounds having seven or less carbon atoms) in stream 11 to yield isomerate recycle stream 10. Isomerate recycle stream 10 is processed in the fractionator 16.
Improving such an energy-intensive process is an active area of research, but it is not a simple matter of optimization of each individual step, as optimization of one step may negatively affect one or more steps in the overall system. Examples of proposed improvements include the following.
U.S. Pat. No. 3,856,874 describes splitting the effluent stream from PX recovery, passing the independent streams over different catalysts, then combining the isomerized streams and recycling.
U.S. Pat. No. 7,439,412 discloses a process for recovering one or more high purity xylene isomers from a C8+-aromatic feed stream including the use of an isomerization unit under liquid-phase conditions. In an example, the product of the liquid-phase isomerization unit is returned to the first fractionation tower in the system. See also U.S. Pat. No. 7,626,065.
U.S. Pat. No. 7,553,998 discloses a process for recovering one or more high-purity xylene isomers from a feed having substantial content of C9+-aromatic hydrocarbons comprising de-ethylation of heavy aromatics followed by fractionation and then passing the stream to a C8-aromatic-isomer recovery to recover high-purity xylene isomers with lowered energy costs. Streams passing through an isomerization unit under liquid isomerization conditions are split, with a portion sent to an isomer recovery unit, and a portion is purged.
WO 2012/058106 and WO 2012/058108 describe processes for producing a PX-rich product, such as (a) providing a PX-depleted stream; (b) providing a parallel configuration of vapor-phase and liquid-phase isomerization units; and (c) splitting the PX-depleted stream and isomerizing the two split streams in the two parallel isomerization units respectively. The process saves energy by reducing the amounts of isomerate recycle from vapor-phase xylenes isomerization which is more energy intensive than liquid-phase xylenes isomerization.
WO 2011/133326 is directed to a xylenes isomerization process, including a liquid-phase isomerization, for the production of equilibrium or near-equilibrium xylenes, wherein the process conditions include a temperature of less than 295° C. and a pressure sufficient to maintain the xylenes in liquid phase that uses at most only ppm levels of hydrogen and that in embodiments can be regenerated numerous times by an in situ procedure.
Other references of interest include U.S. Patent Application Publication Nos. 2008/0262282; 2009/0149686; 2009/0182182; U.S. Pat. Nos. 6,448,459; 6,872,866; and 7,368,620.
Present demand for PX is fairly large and is expected to grow in the future. Consequently, a system maximizing PX production in an energy-efficient manner is highly sought after. While prior attempts to improve PX and optionally OX production abound, most have not been able to overcome the xylenes equilibrium limitation to reduce xylenes recycle. The present inventors have surprisingly discovered a process which overcomes the equilibrium limitation to reduce xylenes recycle by coupling a C9+-aromatics-removal system with an isomerization system or coupling a C9+-aromatics-removal system with a parallel configuration of two isomerization systems. The improved process significantly reduces the energy required and/or increases the production capacity for producing high purity PX and optionally OX.